The present invention relates to fuel cell systems operating on reactant streams that have been enriched by a pressure swing adsorption method. In particular, the present invention relates to solid polymer electrolyte fuel cell systems operating on oxygen enriched air or hydrogen enriched reformate.
Fuel cell systems are currently being developed for use as power supplies in numerous applications, such as automobiles and stationary power plants. Such systems offer promise of economically delivering power with environmental and other benefits.
Fuel cells convert reactants, namely fuel and oxidant, to generate electric power and reaction products. Fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. A catalyst typically induces the desired electrochemical reactions at the electrodes. Preferred fuel cell types include solid polymer electrolyte fuel cells that comprise a solid polymer electrolyte and operate at relatively low temperatures.
A broad range of reactants can be used in solid polymer electrolyte fuel cells. For example, the fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or methanol in a direct methanol fuel cell. The oxidant may be, for example, substantially pure oxygen or a dilute oxygen stream such as air.
During normal operation of a solid polymer electrolyte fuel cell, fuel is electrcchemically oxidized at the anode catalyst, typically resulting in the generation of protons, electrons, and possibly other species depending on the fuel employed. The protons are conducted from the reaction sites at which they are generated, through the electrolyte, to electrochemically react with the oxidant at the cathode catalyst. The catalysts are preferably located at the interfaces between each electrode and the adjacent electrolyte.
Solid polymer electrolyte fuel cells employ a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d), which comprises the solid polymer electrolyte or ion-exchange membrane disposed between the two electrodes. Separator plates, or flow field plates for directing the reactants across one surface of each electrode, are disposed on each side of the MEA.
Each electrode contains a catalyst layer, comprising an appropriate catalyst, located next to the solid polymer electrolyte. The catalyst may, for example, be a metal black, an alloy or a supported metal catalyst, for example, platinum on carbon. The catalyst layer typically contains ionomer that may be similar to the ionomer used for the solid polymer electrolyte (for example, Nafion(copyright)). The catalyst layer may also contain a binder, such as polytetrafluoroethylene. The electrodes may also contain a substrate (typically a porous electrically conductive sheet material) that may be employed for purposes of reactant distribution and/or mechanical support.
In operation, the output voltage of an individual fuel cell under load is generally below one volt. Therefore, in order to provide greater output voltage, numerous cells are usually stacked together and are connected in series to create a higher voltage fuel cell stack. (End plate assemblies are typically placed at each end of the stack to hold it together and to compress the stack components together. Compressive force is generally needed for effecting seals and making adequate electrical contact between various stack components.) Fuel cell stacks can then be further connected in series and/or parallel combinations to form larger arrays for delivering higher voltages and/or currents.
Difficulties may arise with the management of water in a solid polymer fuel cell. For instance, in order to function properly, the ion exchange membrane needs to remain adequately hydrated. However, the inlet reactant streams as supplied may be relatively dry and thus may dry out the membrane in the vicinity of the reactant inlets. Thus, one or both inlet reactant streams may need to be humidified. On the other hand, a substantial amount of product water may be generated at the cathode as a result of the electrochemical reaction therein which can result in flooding downstream in the cathode flow field plate thereby obstructing access of oxidant to the cathode catalyst. As described in U.S. Pat. No. 5,935,726, it may therefore be advantageous to periodically reverse the flow direction of a reactant stream, in particular the oxidant stream, to reduce the likelihood of forming overly wet and overly dry regions in the fuel cell and to reduce or eliminate the need for external humidification of the reactant streams.
For greater output voltages, it is also advantageous to supply fuel cells with concentrated reactant streams and preferably with pure reactant streams (for example, pure hydrogen and oxygen reactants). This is an advantage because the presence of relatively large amounts of non-reactive components in the reactant streams can significantly increase kinetic and mass transport losses in the fuel cells. However, in many applications it may be impractical to store and provide the desired reactants in pure form. For instance, hydrogen gas may be stored in high pressure cylinders, liquefied in a cryogenic container, or alloyed in a metal hydride alloy. Such storage options can all add substantial weight and cost to a fuel cell system. In a like manner, options for storing and providing oxygen gas (for example, in high pressure cylinders or cryogenic containers) also add cost and weight. Instead, hydrogen is frequently obtained by reforming a supply of methanol, natural gas, or the like, on-site or on-board. However, a significant amount of carbon dioxide is also generated in the reforming and it typically becomes a substantial non-reactive component in the reformed fuel stream. Oxygen is typically obtained from the air surrounding the fuel cell system. However, non-reactive nitrogen then typically becomes the major component in the dilute oxidant stream.
Increasing the concentration of the reactant in reformed fuel and/or air streams, that is, enrichment, has thus been considered in the art as a way of improving fuel cell performance. Several enrichment methods are commonly known that involve separating out a component from the reactant stream, including cryogenic, membrane, and pressure swing adsorption methods. In a cryogenic method, component separation is achieved by preferentially condensing a component out of a gaseous stream. In a membrane method, component separation is achieved by passing the stream over the surface of a membrane that is selectively permeable to a component in the stream. In a pressure swing adsorption method, a gas component is separated from a gas stream by preferential adsorption onto a suitable adsorbent under pressure. (The ability of a suitable adsorbent to adsorb a desired gas component is dependent on the partial pressure of that component but also may be dependent on the nature of and partial pressure of any other components present since these other components may also be adsorbed to some extent and/or may interact with the desired component.) The adsorbed component is then subsequently desorbed by reducing the pressure and is removed. By exposing the adsorbent to cyclic swings in pressure, a cyclical adsorption and desorption takes place at the adsorbent, and saturation of the adsorbent may be prevented. The gas stream remaining over the adsorbent (that is, the raffinate) is enriched in the component or components that are not adsorbed by the adsorbent. The gas stream that is later desorbed from the adsorbent (that is, the extract) is enriched in the component that was adsorbed by the adsorbent. Thus, an enriched stream may be derived from either the raffinate or the extract.
In a pressure swing adsorption system however, the desired enriched stream is only provided during one part of the two part pressure swing cycle. Thus, a pressure swing adsorption system typically comprises two portions (or more) of adsorbent in order to provide a continuous stream of enriched gas. The system is operated such that the two adsorbent portions adsorb and desorb the gas component out of phase with each other (that is, one adsorbent portion adsorbs while the other adsorbent portion desorbs during operation). At any given time, enriched raffinate may thus be obtained from the adsorbing portion. Alternatively, at any given time, enriched extract may be obtained from the desorbing portion.
Apparatus for providing an enriched gas stream via pressure swing adsorption typically comprises two chambers, one for each adsorbent portion, and associated plumbing and controls for alternately pressurizing and depressurizing the two chambers and for suitably directing the flow of raffinates, extracts, and the supplied gas stream in a prescribed sequence. In previously described fuel cell applications, pressure swing adsorption apparatus has been incorporated as a separate subsystem between a dilute reactant stream supply (typically a fuel reformate or compressed air supply) and a fuel cell stack or array.
The present methods and systems for enriching reactants for fuel cells employ an integrated pressure swing adsorption apparatus. The pressure swing adsorption method may involve swings in the absolute pressure of a reactant stream or swings in the partial pressure of a reactant stream component or both. Further, temperature swings may also be employed to assist in the adsorption/desorption process.
The operational features of certain fuel cells (for example, solid polymer fuel cells) make them more amenable to integration with pressure swing adsorption apparatus. For instance, fuel cells that normally operate at reactant pressures well above ambient (for example, greater than about 138 kPa (20 psig)) are readily adapted to be able to provide pressure swings of order of the difference between operating pressure and ambient. Such pressure differences may be suitable for useful enrichment via pressure swing adsorption. Thus, means for pressurizing the reactant streams for purposes of pressure swing adsorption and for supply to the fuel cells may be integrated and simplified.
Further, fuel cells that are normally supplied with significant excess reactant (that is, where more reactant is supplied to the fuel cells than is consumed therein) may have a ready supply of somewhat enriched xe2x80x9cwastexe2x80x9d reactant exhaust that can be used for purposes of desorbing and subsequently pressurizing adsorbent in the pressure swing apparatus. For instance, often a significant excess of oxidant may be supplied to the fuel cells. The oxidant stoichiometry (that is, the ratio of the amount of oxidant supplied to that actually consumed in the electrochemical reactions in the cell) may significantly exceed 1 (for example, typically from about 1.5 to 2 in solid polymer fuel cells). Thus, in such an instance, there may be a significant supply of still-enriched oxidant exhaust which may be available to desorb or to augment desorption of adsorbent in the pressure swing apparatus.
Further still, the enrichment method may involve reversing the flow of the reactant stream through the reactant passages in the fuel cells. Thus, the advantages obtained with the use of periodic flow reversal in the fuel cells can conveniently be achieved in combination with reactant enrichment.
Generally, since pressure swing adsorption is more effective at lower temperatures, fuel cell types with relatively lower operating temperature are preferred for purposes of integration with pressure swing adsorption apparatus. Thus, fuel cell systems such as solid polymer fuel cell and alkaline fuel cell systems, with operating temperatures below about 200xc2x0 C., are preferred.
An embodiment of an integrated fuel cell and pressure swing adsorption system comprises the following: at least one fuel cell, a pressurized reactant stream supply comprising a reactant and a non-reactant, and a reactant stream line comprising first and second valves upstream and downstream of the fuel cell and providing a fluid connection through the reactant stream passages of the at least one fuel cell. In this embodiment, the reactant stream line thus provides a path for the reactant stream to flow from the first valve, through the fuel cell passages, and to the second valve and vice versa. The pressurized supply is fluidly connected to both the first and the second valves, and the first and second valves are operative to open and close the reactant stream line between the pressurized supply and the fuel cell. Thus, flow from the pressurized supply can be directed to the fuel cell in either direction through the reactant stream line. The first and second valves may also be operative to vent the reactant stream line thereby providing vents in either flow direction for reactant exhaust from the fuel cell. Additionally, the functions of the first and second valves may be incorporated into a single complex valve that is capable of directing multiple fluid streams.
Embodiments of the fuel cell system may also comprise first and second adsorbent portions for the non-reactant. The adsorbent portions are accessed by the reactant stream in the reactant stream line and may be located external or internal to the fuel cell. The first adsorbent portion may be located either between the first valve and the fuel cell or within the fuel cell itself. The second adsorbent portion may be located between the second valve and the first adsorbent portion. Thus, the sequence of the elements in the reactant stream line of such embodiments is: a first valve, a first adsorbent portion, a second adsorbent portion, and a second valve. The fuel cell is located between the first and second valves in the reactant stream line.
A method for enriching a gaseous reactant stream in the preceding integrated fuel cell and pressure swing adsorption system comprises: alternately directing the reactant stream from the reactant stream supply through the first and second valves, and when the reactant is directed to the fuel cell via the first valve (a) directing the reactant stream through the first adsorbent portion thereby depleting the reactant stream of the non-reactant and enriching the reactant stream in the reactant, and (b) desorbing the non-reactant from the second adsorbent portion; and when the reactant stream is directed to the fuel cell via the second valve (a) directing the reactant stream through the second adsorbent portion thereby depleting the reactant stream of the non-reactant and enriching the reactant stream in the reactant, and (b) desorbing the non-reactant from the first adsorbent portion.
The fuel cell system may comprise more than one fuel cell stack, for example, a first and second fuel cell stack. The first and second fuel cell stacks may however share common end plate and compression mechanisms. With two fuel cell stacks, the method may then further comprise: directing the enriched reactant stream through the reactant stream passages of the first fuel cell stack (but not necessarily through the reactant stream passages of the second fuel cell stack) when the reactant stream is directed to a fuel cell through the first valve, and directing the enriched reactant stream through the reactant stream passages of the second fuel cell stack (but not necessarily through the reactant stream passages of the first fuel cell stack) when the reactant stream is directed to a fuel cell through the second valve. The desorbing of the non-reactant from either or both of the first and second adsorbent portions may be accomplished by reducing the pressure of the reactant stream to ambient in the first and/or second adsorbent portions, respectively (that is, desorption involves a substantial swing in absolute pressure and hence in partial pressure). Preferably, energy is recovered from the pressurized gas in the adsorbent portion as the pressure is reduced to ambient. For instance, gas from an adsorbent portion may be used to drive a turbocompressor as it is vented to ambient.
Alternatively, or in addition, as long as the partial pressure of the non-reactant in the reactant stream exhaust from the fuel cell stack is significantly less than that in the reactant stream supply, the desorbing of the non-reactant may be accomplished by directing the reactant stream exhaust from a fuel cell stack through the adsorbent portions (that is, desorption involves a substantial swing in partial pressure of the adsorbed species but not necessarily a substantial swing in absolute pressure). For instance, the desorbing of the non-reactant from the first adsorbent portion may be accomplished by directing the reactant stream exhaust from the second fuel cell stack through the first adsorbent portion. In a like manner, the desorbing of the non-reactant from the second adsorbent portion may be accomplished by directing the reactant stream exhaust from the first fuel cell stack through the second adsorbent portion. Optionally, both techniques may be employed. For example, the desorbing from each adsorbent portion may involve venting to ambient pressure and purging using the reactant stream exhaust from one of the fuel cell stacks. Such desorbing may be achieved by incorporating additional valve(s) between the two fuel cell stacks in which the valve(s) is operative to vent the reactant stream line and/or to fluidly connect the reactant passages of the two stacks together.
The two adsorbent portions may be located external to the fuel cell stack or stacks. Alternatively, the adsorbent portions may be located within the stack or stacks themselves. For instance, in embodiments comprising two stacks, the first adsorbent portion may be interposed between the first valve and the first fuel cell stack and the second adsorbent portion may be interposed between the second valve and the second cell stack. Alternatively, the first and second adsorbent portions may be located within the first and second fuel cell stacks respectively. In a system consisting of only a single fuel cell, the two adsorbent portions may be located within that fuel cell. In such a case, the adsorbent portion nearest one end of the reactant passage(s) may be adsorbing non-reactant while the adsorbent portion nearest the other end of the reactant passage(s) may be desorbing non-reactant. There need not be a distinct boundary defining a separation between the first and second adsorbent portions (for example, an embodiment comprising a single fuel cell in which adsorbent is distributed along the reactant passage).
There are various locations within a fuel cell stack that are accessible to the reactant stream and thus may be suitable locations for an adsorbent. For instance, the adsorbent portions may be arranged in sub-stacks of their own, thereby forming adsorbent sub-stacks. Alternatively, the adsorbent portions may be arranged in individual adsorbent layers in which an adsorbent layer is associated with one or more membrane electrode assemblies in the fuel cell stacks. Further, the adsorbent portions may be located within the reactant stream manifolds or passages of the fuel cell stacks. In general, because the presence of water may reduce the selectivity of an adsorbent, it may be beneficial to reduce contact between water and the adsorbent by incorporating hydrophobic layers between any adsorbent portions and the reactant stream.
An adsorbent may also be located within a fuel cell stack in or near the cell electrodes. For instance, the adsorbent portions may be located in gas diffusion or porous electrode substrate layers or in sublayers (catalyst support layers) of the membrane electrode assemblies in the fuel cell stacks. Alternatively, the adsorbent portions may be located in catalyst layers of the membrane electrode assemblies. This might be achieved by simply mixing particulate adsorbent with the catalyst in the catalyst layers, or by employing a suitable adsorbent as a support for the catalyst in the catalyst layers. For example, an activated carbon or carbon molecular sieve that selectively adsorbs nitrogen may be considered as such a catalyst support.
In embodiments comprising two fuel cell stacks, the first and second adsorbent portions may be located in a like manner in each of the first and second fuel cell stacks respectively, or not.
The desorbing step in the pressure swing adsorption cycle need not include a venting of the adsorbent to ambient pressure. Desorbing may instead be accomplished by flowing exhaust from a fuel cell stack through the adsorbent portion to be regenerated. This approach may not involve a large absolute pressure swing between adsorption and desorption, but there may still be a substantial partial pressure swing. For instance, an embodiment may be considered wherein the fuel cell system comprises a fuel cell stack in which the first adsorbent portion is interposed between the first valve and the fuel cell stack and the second adsorbent portion is interposed between the second valve and the fuel cell stack. In this embodiment, during adsorption, an adsorbent is directly exposed to the pressurized reactant stream supply, which may have a substantial partial pressure of non-reactant. During desorption, that adsorbent is directly exposed to the still somewhat enriched exhaust from the fuel cell stack which has a substantially lower partial pressure of non-reactant compared to the reactant stream supply. Compared to the reactant stream entering the fuel cell stack, the enriched exhaust will of course be somewhat depleted of reactant.
The method and apparatus may be useful for enriching either or both of an oxidant reactant stream and a fuel reactant stream. For instance, an oxygen enriched reactant stream may be obtained from a pressurized supply of air or a hydrogen enriched reactant stream may be obtained from a pressurized supply of reformate.